Re: RE : fwrite/fread and unbuffered IO

["Joel Sherrill <joel at OARcorp dot com>" <joel dot sherrill at OARcorp dot com>]

Etienne Fortin wrote:
> I created a diff of modifications I made to fread.c of newlib 1.12.0. It
> now works as I expect unbuffered read to work. If the FILE I/O is
> unbuffered, then _read() is directly called with the suplied memory
> pointer and the number of bytes asked for without any other processing.

I don't know the proper behavior on this one.  Jeff?

> By the way, newlib 1.12.0 is working perfectly with RTEMS 4.6.1. The
> only problem is a warning about implicit declaration of _fstat in
> ttyname_r.c of libcsupport. The old header works but not the new of
> newlib 1.12.0. Maybe it's something you are already informed of...

Send me this patch to the list so it can be incorporated into
the newlib cvs.  I also will grab this patch and use it with the
RTEMS RPMs.

If someone approves your other patch, we will use it with RTEMS
as well.

> I'm still not sure if this buffered read thing is the expected behavior
> or not... Anyone please!!! ;)

Have you checked the opengroup.org standard pages?

--joel



> Etienne Fortin
> Sensio
>
>
> 131a132,146
>
> >  /* *************************************** 
> >     AUTHOR: Etienne Fortin (2004-09-17)
> >     Change to accomodate for the fact that
> >     in unbuffered IO, there's no need to
> >     transit data via any kind of buffer.
> >     Because of that, we read into buf
> >     directly for the number of bytes needed.
> >     *************************************** */
> >  if (fp->_flags & __SNBF)
> >    {
> >      return (*fp->_read) (fp->_cookie, (char *) p, total);
> >    }
> >     
> >  /* *************************************** */
>
>
>
>
>
>
> -----Message d'origine-----
> De : Joel Sherrill <joel@OARcorp.com> [mailto:joel.sherrill@OARcorp.com]
>
> Envoyé : 17 septembre, 2004 12:26
> À : Etienne Fortin
> Cc : newlib@sources.redhat.com
> Objet : Re: fwrite/fread and unbuffered IO
>
>
> Is this a termios device by any chance?
>
> Could you try this with read()/write() not their f--- counterparts? That
> would narrow the issue down to either RTEMS or newlib.
>
> --joel
>
> Etienne Fortin wrote:
>
> > Hi everyone,
> > I'm playing with fread and fwrite and found some strange behaviors. 
> > Well, behavior that I wasn't expecting.
> >
> > If I put my FILE in "unbuffered mode" with setvbuf() and I call 
> > fwrite(), the memory pointer supplied to fwrite() is directly used as 
> > the source of the bytes being written to the stream. This is exactly 
> > what I expected since I'm unbuffered, ie no transit via some other 
> > buffer.
> >
> > But with fread, things are quite different. Let's say I'm unbuffered 
> > and I write something like this:
> >
> >    fread(data, 1, 50, fp);
> >
> > I expect fread() to takes the "data" ptr directly as the container of 
> > the received bytes. But strangely, instead, fread() use the 1 bytes 
> > buffer embedded in the FILE structure (_ubuf) as a transit buffer for 
> > the received characters. This is absolutely not the behavior I 
> > expected. If I tell the FILE stream to be unbuffered, I don't expect 
> > my data to transit via some buffer, even if that buffer is as small as
>
>
> > one byte. It's still a buffer. Also, this behavior is quite 
> > inneficient since the driver function responsible for getting 
> > characters from the stream is called the number of bytes needed, in 
> > the example 50 times!!! Unbuffered IO doesn't mean a device can't read
>
>
> > bytes in sequence...
> >
> > Am I missing something here or is this behavior strange?
> >
> > Etienne Fortin
> > Sensio


--
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